Test pads on flash memory cards

ABSTRACT

A semiconductor package is disclosed including test pads formed of solder bumps affixed to the semiconductor package during fabrication. When the package is encapsulated, due to the pressure exerted on the package during the encapsulation process, portions of the solder bumps get flattened out to be generally flush with and exposed on a surface of the semiconductor package. These exposed portions of the solder bumps form the test pads by which the finished package may be tested.

CLAIM OF PRIORITY

This application is a divisional application of U.S. patent applicationSer. No. 11/321,663 filed on Dec. 29, 2005 entitled “Test Pads On FlashMemory Cards”, which application is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate to a flash memory cardincluding an integrated circuit package having test pads.

1. Description of the Related Art

As the sizes of electronic devices continue to decrease, the associatedsemiconductor packages that operate them are being designed with smallerform factors, lower power requirements and higher functionality.Currently, sub-micron features in semiconductor fabrication are placinghigher demands on package technology including higher lead counts,reduced lead pitch, minimum footprint area and significant overallvolume reduction.

One branch of semiconductor packaging involves the use of a leadframe,which is a thin layer of metal on which one or more semiconductor dieare mounted and supported. The leadframe includes electrical leads forcommunicating electrical signals from the one or more semiconductors toa printed circuit board or other external electrical devices. FIG. 1shows a leadframe 20 before attachment of a semiconductor die 22. Atypical leadframe 20 may include a number of leads 24 having first ends24 a for attaching to semiconductor die 22, and a second end (not shown)for affixing to a printed circuit board or other electrical component.Leadframe 20 may further include a die attach pad 26 for structurallysupporting semiconductor die 22 on leadframe 20. While die attach pad 26may provide a path to ground, it conventionally does not carry signalsto or from the semiconductor die 22. In certain leadframeconfigurations, it is known to omit die attach pad 26 and instead attachthe semiconductor die directly to the leadframe leads in a so-calledchip on lead (COL) configuration.

Semiconductor leads 24 may be mounted to die attach pad 26 as shown inFIG. 2 using a die attach compound. Semiconductor die 22 isconventionally formed with a plurality of die bond pads 28 on at leastfirst and second opposed edges on the top side of the semiconductor die.Once the semiconductor die is mounted to the leadframe, a wire bondprocess is performed whereby bond pads 28 are electrically coupled torespective electrical leads 24 using a delicate wire 30. The assignmentof a bond pad 28 to a particular electrical lead 24 is defined byindustry standard specification. FIG. 2 shows less than all of the bondpads 28 being wired to leads 24 for clarity, but each bond pad may bewired to its respective electrical lead in conventional designs. It isalso known to have less than all of the bond pads wired to an electricallead as shown in FIG. 2.

Typically, leadframe 20 is initially formed from a panel including aplurality of such leadframes. The semiconductor die 22 are mounted andelectrically connected to each leadframe in the panel, and theintegrated circuits formed thereby are encapsulated in a moldingcompound. Thereafter, the individual encapsulated integrated circuitsare cut from the panel, or singulated, into a plurality of semiconductorpackages.

It is known to form test pads within the semiconductor package. The testpads typically are exposed to the outside of the package and areelectrically connected internally to one or more of the semiconductordie in the package. After fabrication of a semiconductor package, thepackage may be inserted into a socket on a test card, whereupon the testpads are contacted by probes to test the electrical properties andfunctioning of the semiconductor package to determine whether thefinished semiconductor package performs per specification.

Typically, a pattern for the test pads is formed in a leadframe or othersubstrate such as a printed circuit board during the substratefabrication step. The pattern may for example be formed in a chemicaletching or mechanical stamping process. After formation, the test padsare left exposed during the molding encapsulation step in forming thepackage to allow access to the test pads after package formation. Oncethe package is fabricated and tested via the test pads, the package maybe encased in a pair of mating lids which cover the test pads, andprevent their access while the semiconductor package is in use.

SUMMARY OF INVENTION

The present invention, roughly described, relates to a semiconductorpackage including test pads formed of solder bumps affixed to thesemiconductor package during fabrication. The semiconductor packages maybe formed on leadframes. A plurality of leadframes may be batchprocessed on a panel in a known fabrication process, such as forexample, chemical etching or in a mechanical stamping process usingprogressive dies.

During the package fabrication process, one or more semiconductor dieare mounted and electrically connected to the leadframes to formintegrated circuits. Thereafter, the integrated circuits areencapsulated in molding compound. After encapsulation, the integratedcircuits may be singulated by cutting the integrated circuits from theleadframe panel into a plurality of individual integrated circuitpackages.

The semiconductor die may be formed with a plurality of bond pads. Afterthe semiconductor die are mounted to the leadframe, each bond pad mayreceive a solder bump in a known solder bumping process. Otherelectrically conductive, deformable materials may be used in place ofsolder in alternative embodiments. A portion of the leadframe includingthe contact fingers may lie in a reference plane. The size of the solderbumps is selected so that, once reflowed onto the semiconductor die bondpads, the reference plane 150 runs through the solder bumps and a smallportion of the solder bumps extend beyond the reference plane.

The package is encapsulated so that a bottom surface of the package liesgenerally in the reference plane. Due to the pressure exerted on thepackage during the encapsulation process, the portions of the solderbumps extending past the reference plane get substantially flattened outto be generally flush with the reference plane and the bottom surface ofthe semiconductor package. Thus, the partially flattened solder bumpsare flush with and exposed to the exterior of the package. These exposedportions of the solder bumps form the test pads by which the finishedpackage may be tested.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a conventional leadframe andsemiconductor die.

FIG. 2 is a perspective view of a conventional semiconductor die wirebonded to a conventional leadframe.

FIG. 3 is a panel including a plurality of leadframes.

FIG. 4 is a top view of a single leadframe from the panel shown in FIG.5.

FIG. 5 is a top view of a leadframe including semiconductor die mountedthereon.

FIG. 6 is a top view of a leadframe including solder bumps for formingtest pads according to an embodiment of the present invention.

FIG. 7 is a cross-sectional view through a plane along line 7-7 in FIG.6.

FIG. 8 is a top view of a finished semiconductor package according to anembodiment of the present invention.

FIG. 9 is a bottom view of a finished semiconductor package according toan embodiment of the present invention.

FIG. 10 is a cross-sectional view through a plane along line 10-10 inFIG. 8.

DETAILED DESCRIPTION

The embodiments of the present invention will now be described withreference to FIGS. 3 through 10, which generally relate to asemiconductor package including test pads formed of solder bumps affixedto the semiconductor package. It is understood that the presentinvention may be embodied in many different forms and should not beconstrued to being limited to the embodiments set forth herein. Rather,these embodiments are provided so that this disclosure will be thoroughand complete and will fully convey embodiments of the invention to thoseskilled in the art. Indeed, the invention is intended to coveralternatives, modifications and equivalents of these embodiments, whichare included within the scope and spirit of the invention as defined bythe appended claims. Furthermore, in the following detailed descriptionof embodiments of the present invention, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. However, it will be clear to those with ordinary skill in theart that the present invention may be practiced without such specificdetails.

In general, leadframes used in the present invention would be batchprocessed from a panel of leadframes, such as for example panel 90 shownin FIG. 3. In the embodiment shown in FIG. 3, the panel 90 includes atwo by six array of leadframes 100. It is understood that panel 90 maybe formed in a wide variety of arrays of varying columns and rows inalternative embodiments. As explained hereinafter, integrated circuitsare formed on the plurality of leadframes 100 in panel 90, theintegrated circuits are encapsulated in a protective molding compound,and then the encapsulated integrated circuits are singulated from thepanel to form a plurality of semiconductor packages.

Referring now to FIG. 4, there is shown a single leadframe 100 frompanel 90. Leadframe 100 includes a die paddle 102 for supporting one ormore semiconductor die. Leadframe 100 further includes electrical leads104 for communicating electrical signals to and from one or moresemiconductor die, and contact fingers 106 for transferring electricalsignals between the one or more semiconductor die and an externalelectronic device.

Leadframe 100 may be formed of a planar or substantially planar piece ofmetal, such as copper or copper alloys, plated copper or plated copperalloys, Alloy 42 (42Fe/58Ni), or copper plated steel. Leadframe 100 maybe formed of other metals and materials known for use in leadframes. Inembodiments, leadframe 100 may also be plated with silver, gold, nickelpalladium, or copper.

Leadframe 100 may be formed by known fabrication processes, such as forexample, chemical etching. In chemical etching, a photoresist film maybe applied to the leadframe. A pattern photomask containing the outlineof the die paddle 102, electrical leads 104 and contact fingers 106 maythen be placed over the photoresist film. The photoresist film may thenbe exposed and developed to remove the photoresist from areas on theconductive layers that are to be etched. The exposed areas are nextetched away using an etchant such as ferric chloride or the like todefine the pattern in the leadframe 100. The photoresist may then beremoved. Other known chemical etching processes are known.

The leadframe 100 may alternatively be formed in a mechanical stampingprocess using progressive dies. As is known, mechanical stamping usessets of dies to mechanically remove metal from a metal strip insuccessive steps.

While an embodiment of the present invention is implemented usingleadframe 100 described above, it is understood that the presentinvention may be implemented using other mediums, for example, printedcircuit boards and other substrates or various polymer tapes.

Referring now to FIG. 5, after formation of the leadframe, one or moresemiconductor die 120 may be mounted to the die paddle 102 of leadframe100 to form an integrated circuit. In embodiments where leadframe 100 isused in a Transflash flash memory card, the semiconductor die 120 mayinclude flash memory chips (NOR/NAND) 120 a and/or a controller chip 120b such as an ASIC. However, it is understood that the leadframe 100 maybe used in a variety of semiconductor packages having irregular shapededges, and a variety of different semiconductor chips and devices may beincluded within the completed semiconductor package. The one or moresemiconductor die 120 may be mounted to leadframe 100 in a known mannerusing a dielectric die attach compound, film or tape. Once semiconductordie 120 are fixed to leadframe 100, the die can be wire bonded usingwire 122 to leadframe leads 104 in a known wire bond process.

In embodiments, for example where die 120 b is a controller chip, thechip 120 b may include a plurality of bond pads 124 used to test theoperation of the completed semiconductor package. The bond pads 124 maybe separate from or the same as the bond pads to which wire bonds 122are affixed to connect the die 120 b to the integrated circuit. It isunderstood that the number of bond pads 124 may vary in alternativeembodiments. Bond pads 124 may be redistributed to any desired positionon the chip 120 b using conductive traces (not shown). Redistribution ofthe pads 124 allows sufficient spacing between the pads to receivesolder bumps as explained hereinafter. It is understood that the bondpads 124 need not be redistributed in alternative embodiments.

Referring now to FIGS. 6 and 7, once the semiconductor die 120 aremounted on leadframe 100, one or more of the bond pad 124 may receive amass of solder, such as for example solder bumps 132. Solder bumps 132may be of known solder material, such as lead/Tin (Pb/Sn) or the like.Other possibilities include nickel/Au bumps, epoxy bumps and gold bumps.It is also possible to use solder bumps with polymer cores. It isunderstood that other electrically conductive, deformable materials maybe used in place of solder in alternative embodiments.

The solder bumps 132 may be attached to bond pads 124 using known solderbumping processes, such as for example those used for attaching solderbumps in flip chips and ball grid arrays (BGAs). In such processes, thebond pads 124 may first be preconditioned in an under bump metallurgy(UBM) step to eliminate non-conductive aluminum oxide as is known in theart. The UBM step provides a low and stable contact resistance at thebump-bond pad interface, but the UBM step may be omitted in alternativeembodiments. The solder bumps may then be attached to the bond pads 124in many processes, including evaporation, electroplating, printing,jetting, stud bumping, and direct placement, each of which is known inthe art. In a further embodiment, the solder bumps could be preformedsolder balls affixed to bond pads 124 using force and/or heat as isknown in the art.

As seen in the cross-sectional view of FIG. 7, the leadframe 100 may beformed in two planes. A first portion of the leadframe 100 includingcontact fingers 106 may lie in a plane 150. A second portion of theleadframe supporting the one or more semiconductor die 120 may be spacedfrom the plane 150. The size of the solder bumps is selected so that,once reflowed onto die 120, the plane 150 runs through the solder bumps132 and a small portion of the solder bumps 132 extend beyond the plane150. In embodiments, the solder bumps may have a diameter above thesemiconductor die 120 of approximately 0.5 mm to 1 mm, and moreparticularly, 0.7 mm to 0.8 mm. It is understood that the diameter ofthe solder bumps may vary outside of this range in alternativeembodiments. The solder bumps may extend beyond the plane 150approximately 10 μm to 100 μm, and more particularly, 50 μm to 80 μm. Itis understood that the solder bumps may extend beyond the plane 150 to agreater or lesser degree than set forth above in alternativeembodiments. In embodiments, each of the solder bumps may haveapproximately the same size.

Once the solder bumps 132 have been attached to panel 90, each of theintegrated circuits may be encapsulated with a molding compound 126 asshown in FIGS. 8 through 10. FIGS. 8 through 10 show individualencapsulated semiconductor packages 142 after singulation from the panel90 as explained below. Molding compound 126 may be an epoxy such as forexample available from Sumitomo Corp. and Nitto Denko Corp., both havingheadquarters in Japan. Other molding compounds from other manufacturersare contemplated. The molding compound may be applied according tovarious processes, including by transfer molding, injection molding and,in embodiments, flood molding to form an encapsulation on panel 90encompassing all of the integrated circuits.

In such processes, a panel 90 may be put into a mold having an upper andlower die, or mold caps. In certain embodiments, portions of theleadframe may be left free of molding compound. Such a molding compoundpattern may be achieved by forming a mirror pattern in the upper moldcap. That is, the upper mold cap is formed with a pattern having areasthat contact the panel 90 at portions to be left free of moldingcompound during the encapsulation process. It is understood that theentire panel 90 may be encapsulated in molding compound in alternativeembodiments.

The package is encapsulated so that a bottom surface of the package liesgenerally in the plane 150. In embodiments, the molding machine carryingout the encapsulation process may output an injection force of about 0.8tons to drive the molding compound into the mold cavity. Due to thispressure, the portions of solder bumps 132 extending past plane 150 getsubstantially flattened out to the plane 150 and the bottom surface ofthe semiconductor package. Thus, the partially flattened solder bumps132 are flush with and exposed to the exterior of the package, as shownin the bottom view of FIG. 9 and the cross sectional side view of FIG.10. These exposed portions of solder bumps 132 form the test pads bywhich the finished package may be tested.

The solder bumps 132 are preferably made flush with the surface of thesemiconductor package 142 during the encapsulation process as describedabove. However instead of or in addition to flattening the solder bumpsduring the encapsulation process, one or more additional processes maybe performed to define surfaces in the solder bumps, or otherelectrically conductive, deformable mass, that are flush with thesurface of the semiconductor package 142. Such additional processes maybe performed before and/or after the encapsulation process. Suchadditional processes may include the application of heat, pressure or ashear force to the solder bumps 132 define a surface in the solder bumpsthat is or will be flush with the surface of the semiconductor package142.

Although not critical to the invention, after the molding step, amarking can be applied to the molding compound 126. The marking may forexample be a logo or other information printed on the surface of themolding compound for each integrated circuit. The marking may forexample indicate manufacturer and/or type of device. The step of markingis not critical to the invention and may be omitted in alternativeembodiments.

After encapsulation and marking, each of the encapsulated integratedcircuits in panel 90 may then be singulated, by cutting the integratedcircuits in panel 90 into a plurality of individual integrated circuitpackages 142. Known cutting devices include, for example, water jetcutting, laser cutting, water guided laser cutting, dry media cutting,and diamond coated wire. Water jet cutting may be the preferred cuttingdevice given its small cutting width and its rapid cutting rate. Watercan also be used together with laser cutting to help complement or focusits effects. A further description of the cutting of integrated circuitsfrom a panel and the shapes which may be achieved thereby is disclosedin published U.S. Application No. 2004/0259291, entitled, “Method ForEfficiently Producing Removable Peripheral Cards,” which application isassigned to the owner of the present invention and which application hasbeen incorporated by reference herein in its entirety. It is understoodthat the singulated integrated circuits may be formed by other processesthan that described above in alternative embodiments.

In the embodiments shown, the package 142 may for example be used in aTransflash flash memory device or Micro SD card. However, it isunderstood that the package 142 may be cut into a variety of shapes andsizes, including rectangular, in further embodiments.

In an embodiment, twenty-one test pads are provided. It is understoodthat the package 142 may alternatively include twenty-seven solder bumps132 forming the test pads, and may alternatively include more or lessthan twenty-one or twenty-seven, for example one or more test pads.Moreover, while the solder bumps 132 are disclosed above as beingmounted directly onto bond pads 124 on the controller chip 120 b, it isunderstood that the solder bumps 132 forming the test pads in thefinished package may be provided on the memory chip 120 a, or on theleadframe 100, and connected to the appropriate bond pad with the use ofconductive traces and/or bond wires.

Once the package is fabricated and tested via the test pads, the packagemay be encased in a pair of mating lids which cover the test pads, andprevent their access while the semiconductor package is in use. Thepackage may operate without lids in further embodiments. In suchembodiments, the test pads may be covered using for example a maskinglabel, epoxy or ink. It is also conceivable that the test pads be leftuncovered.

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A semiconductor package, comprising: a substrate; a semiconductor dieaffixed to the substrate; a bond pad formed in the semiconductor die;solder provided on the bond pad capable of use as a test pad for testingthe electrical properties of the semiconductor die; and molding compoundfor encapsulating at least the semiconductor die; the semiconductorpackage formed by the steps of: (a) coupling solder to the bond pad; (b)encapsulating the semiconductor die in a molding compound, said step (b)of encapsulation resulting in a surface of the semiconductor packagebeing defined; and (c) deforming at least a portion of the solder,during said step (b) of encapsulating the semiconductor die in themolding compound, to define a surface of the solder that issubstantially flush with the surface of the semiconductor packagedefined in said step (b).
 2. A semiconductor package as recited in claim1, wherein the substrate is a leadframe.
 3. A semiconductor package asrecited in claim 1, wherein the substrate is a printed circuit board. 4.A semiconductor package as recited in claim 1, wherein the semiconductordie is a controller chip used in a flash memory device.
 5. Asemiconductor package as recited in claim 1, wherein the semiconductorpackage is configured for use as a flash memory device.
 6. Asemiconductor package as recited in claim 1, wherein the solder is asolder bump.
 7. A semiconductor package as recited in claim 1, whereinthe solder is a solder ball.
 8. A semiconductor package as recited inclaim 1, wherein said step (a) of coupling solder to the bond pad isperformed by at least one of the following processes: evaporation,electroplating, printing, jetting, stud bumping, and direct placement.9. A semiconductor package as recited in claim 1, wherein the solder isa solder ball and said step (a) of coupling solder to the bond padcomprises the step of coupling the solder ball by at least of the stepsof applying heat and pressure.